1. Field of the Invention
The present invention relates to an image forming apparatus with a speed control function. More particularly, the present invention relates to the image forming apparatus which executes speed control to reduce speed variation of a tandem MFP (Multi-Function Product/Printer/Peripheral).
2. Detailed Description of the Related Art
A tandem MFP configured to form a full color image typically includes a transfer belt and image forming units. The image forming units electrophotographically form different single colored toner images, respectively, to form a full color image by superimposing these toner images one over another on the transfer belt. The full color image is transferred from the transfer belt to a sheet.
The transfer belt is wound around a drive roller and an idle roller in many cases. The drive roller rotated by a motor causes the transfer belt to run. The running speed of the transfer belt varies depending on various factors (e.g. accuracy in deceleration of a decelerator, accuracy in diameter of the drive roller, variation in thickness of the transfer belt, and expansion/contraction of the transfer belt). Accordingly, it is not sufficient to keep the running speed of the transfer belt at desired speed only by keeping rotating speed of the motor constant.
A specific image forming apparatus detects an error, which relates to running speed of a transfer belt, resulting from variation in thickness of the transfer belt. The detected error is used for the speed control of the transfer belt. In order to detect the error in the running speed of the transfer belt, a speed detection roller is typically used. The speed detection roller rotates as the transfer belt runs.
The dimensional accuracy of the speed detection roller (e.g. accuracy in diameter and eccentricity) directly affects the error detection for the running speed of the transfer belt. The above image forming apparatus does not make any correction for the dimensional accuracy of the speed detection roller. Accordingly, feedback control for the running speed of the transfer belt is executed based on a detection amount including a reading error generated during one revolution of the speed detection roller. Thus, according to the prior art, variation in the running speed of the transfer belt resulting from the reading error of the speed detection roller is newly generated. Alternatively a speed detection roller is very accurately fabricated in order to reduce the effect from the reading error of the speed detection roller.